Remove `PLUGIN` from component class log level environment variables

Remove `PLUGIN` from component class log level environment variables

For example, `BABELTRACE_PLUGIN_CTF_FS_SRC_LOG_LEVEL` becomes
`BABELTRACE_SRC_CTF_FS_LOG_LEVEL`. This is in line with the way we
specify the component classes in the CLI and documentation, for
example `src.ctf.fs`.

The log level environment variables common to a whole plugin are still
prefixed with `PLUGIN`, for example
`BABELTRACE_PLUGIN_CTF_METADATA_LOG_LEVEL`.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>